LIMERICK — In an effort to prevent hydrogen explosions like the one that destroyed parts of Japan’s Fukushima nuclear plant two years ago, the government is requiring 31 U.S. nuclear plants with a similar reactor design to install an upgraded ventilation system in six years.

The order, issued June 6, applies to GE Mark I and Mark II Boiling Water Reactors. The two nuclear reactors at Exelon Nuclear’s Limerick Generating Station are of the Mark II design.

The new order “contains more specifics on what Mark 1 and 2 Boiling Water Reactor (BWR) owners must do to meet our new requirements for reliable hardened vents,” Nuclear Regulatory Commission, spokesman Neil Sheehan wrote in an e-mail to The Mercury.

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The requirements “include having vents capable of handling the pressures, temperatures, hydrogen concentrations and radiation levels that would be expected to result from a damaged reactor. The enhancements also ensure plant personnel can operate the vents safely if the reactor core is damaged,” he wrote.

“The accident at the Fukushima Dai-ichi power station reinforced the importance of reliable operation of containment plants for vents with Mark I and Mark II containments,” Eric J. Leeds, director of the office for nuclear regulation for the NRC, wrote in a June 6 order.

The explosions at Fukushima occurred during the earthquake and tsunami-fueled disaster that rocked northern Japan in March, 2011.

In citing the need for the new regulations, NRC wrote “the events at the Fukushima Dai-ichi power plant following the March 2011 earthquake and tsunami highlight the possibility that such events such as rare natural phenomena could challenge the traditional defense in-depth protections related to preventing accidents, mitigating accidents to prevent the release of radioactive materials and taking actions to protect the public should a release occur.”

The NRC explained that “these problems with preventing the contaminants under the challenging conditions following the tsunami, contributed to the progression of the accident from inadequate cooling of the core leading to core damage, to compromising containment functions from over-pressure and over-temperature conditions and to the hydrogen explosions that destroyed reactor buildings (and secondary containment) of three of the Fukushima Dai-ichi units.”

This led to the release of radioactive materials which further hampered efforts to bring the plant under control.

The “hardened venting” the NRC is requiring is intended to not only allow the build-up of hydrogen gas to be vented to the exterior, and thus prevent the build-up of pressure that causes the explosions, but also to be operable should the plant lose power during a severe accident.

“The enhanced order requires the vents to handle the pressures, temperatures, hydrogen concentrations and radiation levels from a damaged reactor. The enhancements also ensure plant personnel can operate the vents safely if the reactor core is damaged,” Sheehan wrote.

The venting requirements are just one facet of a broader reaction by the NRC and nuclear plant operators, all of which are hoping to learn lessons from the Fukushima accident.

“More than two years after the events at Fukushima, Exelon has taken numerous steps to apply key learnings to ensure the continued safe operation of Limerick Generating Station,” site communications manager Dana Melia wrote in an email responding to a request for comment from The Mercury.

“The specifics of the recent order have been part of ongoing discussions with the NRC and we are well prepared to meet all requirements including adding hardened containment vents at Limerick. These enhancements will be made during refueling outages and will not affect our ability to continue to create clean, safe and reliable power for the region,” Melia wrote.

The improvements are to be made in phases, with the first phase completed no later than June, 2018, and the second phase a year later.

About the Author

Evan Brandt has worked for The Mercury since November 1997. His beat includes Pottstown, the surrounding townships and the Pottstown and Pottsgrove school districts, as well as other varied general topics like politics, the environment and education. Reach the author at ebrandt@pottsmerc.com
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